The tomato yellow leaf curl virus (hereinafter occasionally referred to as “TYLCV”) is a type of relatively novel plant virus that was discovered in Israel in 1964.
In Japan, cases of tomato yellow leaf curl disease caused by TYLCV were discovered concurrently in 1996 in Nagasaki Prefecture, Aichi Prefecture and Shizuoka Prefecture. Tomato yellow leaf curl disease then spread rapidly in areas of greenhouse tomato production. Particularly extensive outbreaks have occurred since 2000 in Kyushu region, which is a main production area of fresh market tomato, and on many farms all of the cultivated tomatoes have been damaged by TYLCV. In each prefecture, farmers have been warned to take special care, and intense efforts have been made to prevent TYLCV by pesticide application and the like, but TYLCV damage still continues.
Symptoms of tomato yellow leaf curl disease begin with yellowing of tomato leaves, which subsequently become deformed as the leaf edges gradually curl under. When symptoms are severe, leaves of an entire tomato plant appear like permed hair. Symptoms do not extend to fruit, but if tomato is infected with TYLCV at an early stage of growth, only up to about the second bunch of fruit will be set, causing serious losses amounting to a 70% to 80% reduction in yield.
Tomato yellow leaf curl disease becomes persistent and pervasive when it is transmitted by the whitefly Bemisia tabaci (Gennadius), which is an insect vector of TYLCV.
Meanwhile, there are no effective antiviral agents against plant viruses. Up until now, the most common methods of controlling plant viruses have been by applying pesticides to kill insect vectors that transmit viruses, using insect-proof nets and repellent materials to physically prevent entry of insect vectors to cultivation facilities, and sterilizing soil, removing infected plants, sterilizing cultivation tools, using barrier crops, and planting virus-tolerant crops.
The same applies to control of TYLCV, and main methods used for disrupting infection cycle of TYLCV involve controlling Bemisia tabaci which is an insect vector of TYLCV and removing infected plants at an early stage (Non-patent Document 1).
However, if insect-proof nets with a mesh of 0.4 mm or less effective for prevention of entry in order to control Bemisia tabaci are used, it is concerned about temperature elevation in cultivation facilities and thus Bemisia tabaci control using insect-proof nets is difficult to implement in the field.
Moreover, a variety of tomato cultivars with different growing seasons are grown in Kyushu region which is a major tomato producing area, so that tomatoes are being grown somewhere at every time of year. As a result, Bemisia tabaci carrying TYLCV is not killed off by winter cold because Bemisia tabaci moves between outdoor farms and indoor facilities according to a variety of tomato cultivars, making TYLCV control difficult because there is no interruption in infection cycle of TYLCV.
Moreover, an insecticide-tolerant Bemisia tabaci referred to Bemisia tabaci biotype Q (hereinafter occasionally referred to as “Bemisia tabaci Q”) has recently begun to proliferate, restricting usefulness of pesticide control.
Looking at TYLCV tolerant tomatoes, genes such as Ty-1, Ty-2, Ty-3 have been discovered in wild tomatoes. When tomatoes have these genes, virus concentrations are reduced within the tomatoes and symptoms are suppressed, but TYLCV infection itself is not arrested (Non-patent Documents 2 and 3).
Tomatoes having these genes introduced by hybridization have already appeared on the market, but due to the nature of these genes, it is known that in all cases the tomatoes are liable to infection by TYLCV and then viruses proliferate inside the tomatoes (Non-patent Document 4).
Therefore, if Bemisia tabaci control is neglected when tomatoes with these introduced genes are cultivated, the tomatoes will carry TYLCV even if symptoms by TYLCV are suppressed, and will become sources of TYLCV infection, placing any susceptible tomato varieties in the surrounding area at serious risk of TYLCV infection.